Reducing CO2 efficiently by light energy like plants has been required from the standpoint of energy problems and environmental concerns. Plants use a system called the Z-scheme to excite light energy in two stages. Using a photochemical reaction of this system, plants synthesize cellulose and sugar by obtaining electrons from water (H2O) and reducing carbon dioxide (CO2).
However, few technologies that can efficiently dissolve CO2 with electrons obtained from water through artificial photosynthesis without using any sacrificial reagent are available.
A photochemical reaction device disclosed in JP-A-2011-094194, for example, has an oxidation reaction electrode for producing oxygen (O2) by oxidizing H2O and a reduction reaction electrode for producing a carbon compound by reducing CO2. The oxidation reaction electrode has an oxidation catalyst for oxidizing H2O on a surface of a photocatalyst and gains potential with light energy. The reduction reaction electrode has a reduction catalyst for reducing CO2 on a surface of the photocatalyst and is connected to the oxidation reaction electrode with an electric wire. The reduction reaction electrode reduces CO2 to produce formic acid (HCOOH) by gaining reduction potential of CO2 from the oxidation reaction electrode. To gain the potential necessary for reducing CO2 using an optical wavelength and a photocatalyst, the photochemical reaction device thus employs a Z-scheme-type artificial photosynthesis system that imitates plants.
However, JP-A-2011-094194, the solar energy conversion efficiency is very low at around 0.04%. This is because the energy efficiency of the photocatalyst excited by the optical wavelength is low. Because the reduction reaction electrode is connected with the oxidation reaction electrode with an electric wire, the efficiency in extracting electricity (electric current) decreases due to interconnection resistance, and, as a consequence, the efficiency becomes low.
An device that has a configuration to produce a reaction by catalysts disposed on both sides of a silicon solar cell used for achieving the reaction potential is disclosed in JP-A-H10-290017. S. Y. Reece, et al., Science. vol. 334. pp. 645 (2011) describes an device which includes layered silicon solar cells for achieving the reaction potential and produces an electrolysis of H2O by disposing catalysts on both sides of the silicon solar cells. Both of these devices have a very high solar energy conversion efficiency of 2.5%.
These devices are easily configured in a large size because they do not need to be hard-wired. They also have another feature in which a material partition process is not necessary because the cell itself plays a role of a divider plate to insulate materials.
These devices, however, have not succeeded in the reduction reaction of CO2. Such plate-like laminate structure moreover does not take into consideration the fact that, for the CO2 reduction reaction, ions with a positive electric charge produced at the oxidation side and ions with a negative electric charge produced at the reduction side need to move to the opposite sides. In an oxidation-reduction reaction in which H2O is used as an electron donor instead of a sacrificial catalyst, in particular, proton (hydrogen ion (H+)) movement is indispensable.
A CO2 dissolution technology that uses light energy and has a high photoreaction efficiency thus needs to be developed.